This invention relates to a solenoid actuator useful for application to hydraulic valves and to a valve arrangement incorporating such an actuator.
Fluid power systems often rely upon solenoid-actuated valves to control the flow of fluid. It is often advantageous to be able to switch fluid from one path to another as fast as possible, such that the time spent in intermediate positions is minimised, hence minimising energy losses caused by pressure drops though the valve.
Often such valves are constructed as single acting solenoids, whereby a ferromagnetic sliding member such as a spool or a poppet is attracted to an end face of a solenoid, the return flux being passed into the ferromagnetic member in a direction transverse to the axis of the solenoid, such that flux flowing in the circuit produces a net axial force on the moving member which moves it from one position to another. Usually the solenoid cannot produce a force acting in the opposite sense so this force is provided by a spring or some component of the fluid pressure. Such valves often have transit times in the direction of actuation of the order of 40 ms.
Hydraulic/pneumatic pumps and motors are referred to herein as “fluid-working machines”. A new class of such machines is emerging in which the commutation of the working chambers is provided not by mechanical means such as port plates, but by solenoid-actuated valves controlled by a digital computer. This technique allows such a machine to displace fluid in discrete units, and the applicant's machines are therefore termed “Digital Displacement™”. Operators of these pumps wish to drive them directly from the shafts of industrial diesel engines, which run in the range 1800-2800 rpm. In order to achieve these speeds the commutating valves must actuate many times each second. Actuation time should be kept below 5 ms for accurate commutation.
Solenoid valves according to the prior art cannot achieve this speed of actuation. Usually there is a restoring force to keep the armature in the original position, which is the default position if the coil is inactive. Before the armature moves, the coil must be charged with current, which, because of the high inductance of the coil, takes many milliseconds—this is termed the latency of the coil. Force builds on the armature gradually, until it exceeds this restoring force and causes acceleration of the armature towards the second position. The initial acceleration is low as the force builds gradually, due to the long time constant of the coil. These effects cause a long valve transition time.
Because the period during which the armature is in motion is long, and the latency of the coil is long, there is much uncertainty about the exact time when the valve reaches its actuated position.